iv PLAGIAUJLAX 99 Theromorphous group). (2) The Triconodonta, which were Marsupials, though in all probability with a complete succession of teeth and with an allaritoic placentation. This group will include the genera Phascolotherium and Amphilestes, as well as Triconodon and SpalacotJieriufn. Finally we have (3) the Tritu- berculata (or Insectivora Primitiva) with the genera A-inpliiiheriuyn, Perarnus, AmblotTierium, Stylacodon, and Dryolestes. We shall take these three groups in order. The Multituberculata have already been to some extent denned, if such a word can be used to express the summation of the very scanty information at our disposal. Of this group, Plagiaulax is a genus which occurs in the Purbeck beds; it is only known by lower jaws implying an animal of the size of a Eat or rather smaller. The jaws have in front a large incisor which looks Kodent-like, and also like those of the Diprotodont Marsupials ; but it is held that these teeth did not grow from persistent pulps, and there is in any case no anterior thickened coating of enamel. Canines are absent; the diastema is followed by four premolars increasing progressively in size and possessing somewhat complicated grinding surfaces. These surfaces are formed by several obliquely-set ridges. The succeeding teeth are termed molars on account of their difference in structure, and bhere are but two of them on each side. The molars are of a pattern common in the Multituberculata; the centre is hollowed, ind the raised rim is beset by tubercles. Other Jurassic genera >f Multituberculates are Bolodon, Allodon, and titereognathus. ^Lll of these possess the same rnultituberculate molars. Of the Triconodonta the type-genus is Triconodon. This genus s better known than most Jurassic mammals, since both the upper tnd the lower dentition have been described. It appears to have >ossessed the typical Eutherian dentition of forty-four teeth, to rhich a fourth molar is added in some species. The great differ- nce between the molars and premolars argues a complete tooth- hange. The genus is American as well as European. Spala^otJieriu'm has more molars, five or six. Phascolotherium 'bucklandi, on the other hand, is a much Ider type in the form of its teeth. There are, however, not so lany of them as in AmpMtherium ; PJiascolptherium has but two remolars and five molars, making a total of forty-eight teeth. lhe teeth are of the triconodont form, the three cusps being in ne, and the middle one the largest.